This invention relates to electrical resistance furnaces and particulary to a resistance furnace utilizing counteracting gas flows and a method for drawing fibers utilizing such a furnace.
A prior art method for drawing of silica fibers utilizing an electrical resistance furnace includes the use of a thin ceramic muffle tube to protect heater elements, usually graphite elements or tungsten wires, within the furnace from exposure to atmospheric oxygen and to enable a silica preform and a fiber drawn from the preform to be heated in an atmospheric environment within the muffle tube. The muffle tube is expensive and requires expensive controls and alarms to prevent it from being exposed to rapid temperature variations which tend to crack and break the tube. To prevent the tube from cracking, each furnace start-up and shut-down operation takes at least two to three hours. An alternative to cooling the furnace down to room temperature between production runs is to keep the furnace at some elevated temperature, which results in consumption of electric power.
Furthermore, as high furnace temperatures contaminants evaporate from the muffle tube, heater elements and a heat resistant metal shield lining the furnace. Most of the contaminants evaporated by the heater elements and the metal shield diffuse through the muffle tube. A great portion of these contaminants along with contaminants evaporated from the muffle tube are carried away by convectional air currents within the muffle tube, however, some contaminants condense on the preform and the fiber, contaminating both. Introduction of a forced flow of air or gas into the muffle tube, to aid the convectional air currents in the flushing out of contaminants, causes temperature gradients in the tube material. The gradients eventually cause the tube to crack. Thus, it is expensive, time consuming and difficult to draw uncontaminated fibers with a furnace wherein the resistance elements of the heater are protected from oxidation by a muffle tube.
Therefore, it is an object to provide a method for drawing of silica fibers in an electrical resistance furnace without a muffle tube.
Another object is to provide a low cost method for drawing silica fibers in an electrical resistance furnace.
Yet another object is to provide an electrical resistance furnace without an expensive control and alarm system.
These and other objects are realized in an illustrative method for drawing fibers which method has the following steps. First, the preform is heated in a furnace so that the preform begins to flow forming a fiber. Second, the fiber is pulled from the furnace; and third, the fiber and the preform are flushed by counteracting inert gas flows protecting the preform and the fiber from contamination.
A feature of the invention is the flushing of the preform and the fiber within the furnace with counteracting gas flows, the gas flows protecting the preform and the fiber from contamination.
Another feature of the invention is the injection of gas from opposite ends of the furnace into the chamber and exhaustion of the gas from the chamber through an exhaust located near a heater.
Yet another feature of the invention is the application by the counteracting gas flows of an inert gas.